The present invention relates to a substrate used for producing a semiconductor device, a method for producing the substrate, a photoelectric conversion device and a method for producing the photoelectric conversion device. Particularly, the present invention relates to a semiconductor-device-producing substrate in which substantially spherical granular crystals are arranged in the form of a plurality of layers on a substrate and in which the granular crystals are mechanically and electrically connected to one another and the granular crystals abutting on the substrate are mechanically and electrically connected to the substrate to form a PN junction on the granular crystals to thereby provide large-area active layers and such a substrate, further relates to a method for producing such a substrate, a photoelectric conversion device using such a substrate, and a method for producing such a photoelectric conversion device.
In recent years, semiconductor wafers, especially silicon or compound semiconductor wafers are used popularly for producing semiconductor devices such as a solar cell, a rectifier, a light-emitting diode, a transistor, an integrated circuit, etc.
Semiconductor substrate crystal wafers having accurately mirror-polished surfaces are mainly used for production of these semiconductor devices. Ensuring of materials and extreme reduction in cost have become more important with the improvement of the quality of substrate materials and mass production because of the advance of high-density integration of semiconductor devices and the increase in size of substrate crystals.
The related art will be described below about production of a solar cell (photoelectric conversion device) using silicon by way of example.
Several methods have been used in the related art for production of a solar cell. As examples of the method using a flat substrate, in the related art, there are known: (1) a method in which a PN junction is formed on a surface of a flat silicon monocrystal or polycrystal by a diffusion method or a chemical vapor deposition (CVD) method; (2) a method in which P type and N type amorphous silicon layers are laminated on a flat metal plate to thereby form a PN junction; (3) a method in which a laminate structure having two layers of PN junctions series-connected through a tunnel junction is formed on a flat substrate (Appl. Phys. Lett. 65 (8) Aug. 22, 1994, p989); etc.
As examples of the method using granular crystals, there are known: (4) a method in which P type spherical grains each having a PN junction surface are inserted one by one into holes formed in aluminum foil having opposite surfaces coated with a polymer material to thereby connect electrodes of an N type portion of the surface and then a part of the N type layer of the spherical grains is removed to take out the electrodes from a P type portion of this part to thereby form a spherical grain array having a PN junction (JP-A-58-54684); (5) a method in which a plurality of P type granular silicon crystals are arranged on an aluminum thin film on the substrate, the whole surface of the aluminum thin film is coated with silicon dioxide by CVD, silicon dioxide films on the surfaces of the granular crystals are removed selectively, and n-type impurities are added to the surfaces of the granular crystals or Schottky barriers of tin oxide film are formed to thereby form a solar cell (JP-A-51-27077 and JP-A-51-129192); etc.
FIGS. 5A to 5D are views for explaining a method for producing a photoelectric conversion device (solar cell) in the related art. A technique of the related art will be described below with reference to FIGS. 5A to 5D. That is, FIGS. 5A to 5D show an example of the method (5) using granular crystals. In FIGS. 5A to 5D, the reference numeral 11 designates silicon fine crystals; 12, a substrate; 13, an aluminum thin film layer; 14, an alumina thin film; 15, a silica (SiO.sub.2) thin film; 16, a surface electrode; and 17 and 17', lead wires.
First, as shown in FIG. 5A, silicon fine crystals 11 formed from silane halide by vapor-phase fluidization are deposited on an aluminum thin film layer 13 formed on a metal or ceramic substrate 12. Then, the resulting substrate is heated at a temperature near the eutectic point 575.degree. C. of aluminum and silicon, so that the silicon fine crystals 11 are fixed to the aluminum thin film layer 13. Further, the resulting substrate is oxidized so that alumina thin films 14 are formed on the aluminum thin film layer 13 and silica thin films 15 are formed on the silicon fine crystals 11 as shown in FIG. 5B.
Then, the silica thin films 15 on the silicon fine crystals are selectively removed with a hydrofluoric acid so that the silicon fine crystals 11 are exposed as shown in FIG. 5C. N type impurities are introduced in the silicon fine crystals 11 through the exposed surfaces of the silicon fine crystals 11 to provide a thin-film metal electrode as a surface electrode 16 or provide a transparent electrode of tin oxide, or the like, and form Schottky junctions between the electrode 16 and the silicon fine crystals 11. The lead wires 17 and 17' are led out from the surface electrode 16 and the aluminum thin film 13 respectively. In the aforementioned manner, a solar cell is produced.
In the related-art methods (1) and (2) using a flat substrate, silicon monocrystal, silicon polycrystal or amorphous is used as a material. The photoelectric conversion efficiency per unit area of the substrate is in a range of from about 7% to about 15%. Further, semiconductor-grade high purity crystal is used as a substrate material. Accordingly, there arises a problem that it is difficult not only to reduce the cost but also to secure a large quantity of material. In the related-art method (3) in which a laminate structure having two layers' PN junctions series-connected through a tunnel junction is formed on a flat substrate, the photoelectric conversion efficiency exceeds 20% but the producing method is complex. Accordingly, there arises a problem that it is difficult to perform mass production and to reduce the cost.
In the related-art method (4) using granular crystals, not only photoelectric conversion is performed only by single sides of granular balls inserted in holes of the aluminum foil plate but also the distance between adjacent granular balls is large. Accordingly, there arises a problem that the conversion efficiency per unit area is low to be in a range of from 8% to 10%. In the related-art method (5), the lower surfaces of the silicone fine crystals arranged on the aluminum thin film are alloyed with aluminum to serve as an electrode, so that photoelectric conversion is performed only by the residual surface layer. Accordingly, in the related-art technique, the PN junction area per unit area of the substrate for performing photoelectric conversion is no more than twice the substrate area. Furthermore, the surfaces of the silicon fine crystals obtained by granulation in a vapor-phase reaction of a silane group are rough and the fine crystals contain hydrogen or chlorine gas and have high specific resistance (not smaller than 100 .OMEGA..multidot.cm). Accordingly, there arises a problem that sufficient photoelectric conversion property cannot be obtained.